For many applications it is desirable to transmit video images to remote locations via a telecommunications network. Such video images are often stored in the form of a digital video signal or sequence in a storage medium such as a CD-ROM or digital tape. Typically, a digital video sequence to be transmitted to a remote location via a telecommunications network is read out of the storage medium in a high resolution interlaced format such as the CCIR 601 format. The CCIR 601 format utilizes a Y, U, V representation of a video sequence. There are 30 frames per second with each frame comprising two interlaced fields. The field size of the Y (luminance) component is 720.times.240 pixels per field. The field sizes of the U and V (chrominance) components are 360.times.240 pixels per field.
Because of bandwidth limitations in the telecommunications network, it is desirable to compress a video sequence to be transmitted to a remote location so that the video sequence occupies a bandwidth of about 1 Megabit/sec. Such compression is accomplished in two phases, a preprocessing phase and an encoding phase.
In the first or preprocessing phase, a stored video sequence is converted from a high resolution interlaced format such as the CCIR 601 format to a lower resolution progressively scanned format such as the CIF format. The CIF format utilizes a Y, U, V representation of a video sequence. There are 30 progressively scanned frames per second, with the Y component having a frame size of 360.times.240 pixels and the U and v components having a frame size of 180.times.120 pixels. Thus a frame in the CIF format has one-fourth the number of samples as a frame in the CCIR 601 format.
After conversion to the CIF format, during the second or encoding phase, the video sequence is compression encoded for transmission to a remote location. Illustratively, the compression encoder utilizes temporal predictive coding with motion compensation and transform coding as is found in the CCITT RM8 coding loop (see "Description of Reference Model 8 (RM8)", Document 525, CCITT SG XV, Working Party XV/4 Specialist Group For Visual Telephony).
A preprocessing phase involving conversion between a high resolution interlaced CCIR 601 video sequence and a lower resolution progressive CIF video sequence is utilized because the high resolution interlaced CCIR 601 sequence is not suitable for compression encoding.
Conventionally such conversion is performed by dropping every other field in the interlaced sequence to form an intermediate sequence and dropping every other column in the intermediate sequence. While this conventional conversion procedure is easy to implement, it has serious drawbacks In particular, the resulting progressive CIF sequence suffers from aliasing in the vertical and temporal directions and is noisy. Thus, when a CIF sequence is formed in the foregoing manner and compression encoded for transmission to a remote location, a high quality video image, i.e., an image with VCR quality, cannot be satisfactorily reconstructed at the remote location.
Accordingly, it is an object of the present invention to provide an improved preprocessing algorithm and circuit for converting between high resolution interlaced format such as CCIR 601 and a lower resolution progressive format such as CIF, so as to enable video images to be compression encoded and remotely transmitted with improved quality.